1. Field of the Invention
The present invention relates generally to rotary machines, and more particularly, to positioning the rotor apices of cartiodal rotary machines having two or more lobes and one or more cartiodal projections in the housing.
2. Description of the Related Art
The positioning mechanism for the rotor of cartiodal rotary machines essentially steps down the relative rotational speed of the rotor. The speed reduction is in a 1:2 ratio for the two-lobe machine, 2:3 ratio for the three-lobe rotor, and so forth up to any number of lobes. The mechanisms are numerous, the best known probably being the internal gear of the Wankle or Mazda engine that has a pitch diameter of 1.5 times the pitch diameter of the fixed spur gear.
An important consideration is the passage of the shaft through the positioning mechanism for this class of rotary machine. There are several advantages to this. The first is in that the shaft can be supported on both sides of the rotor. Another advantage is that the alignment for the positioning mechanism is much better when the shaft rotates in a bearing positioned near the mechanism. This becomes of greater consideration when the rotor is long relative to the stroke. Finally, for a rotor that is supported by a bearing only on the side having the positioning element, one end of the rotor can be capped, which eliminates the need for any moving seals on the unsupported side of the rotor. The rotor can essentially be tapered to a small distance between apexes and twisted around to shapes designed for improved flow of liquids or gases.
Rotary machines of this type have a longer stroke relative to the overall size of the rotor, and behave somewhat differently than those with shorter strokes. The rotor occupies a much smaller volume of the chamber and consequently has much less mass, while the shaft is also of lighter construction. These characteristics, although for the most part a disadvantage for high power density of the flow medium, are of significant benefit for lower overall pressure applications.
A sizeable gap between the rotor tips and chamber wall for the machine can still allow effective operation as an expansive device for lower pressure ratios and high RPM. The pressure ratio thus will have smaller variations over a high range of RPM when compared to turbo machinery. The positive displacement machine applied to increasing inlet pressure of an engine is beneficial in this respect, but has the disadvantage of not using the exhaust energy.
The use of a positive displacement machine in place of an exhaust turbine for an engine in concept is recognized as being possible. The obstacles to employing such a device are numerous. It is helpful to clarify these by drawing a contrast to the turbocharger. There are two fundamental schemes for the employment of a positive displacement device as opposed to the one familiar fundamental scheme employed with the exhaust turbine.
The exhaust turbine is connected to the outlet of the engine's cylinders by a tube that has a volume contained within. The exhaust turbine expands the exhaust gases from the engine and to some degree converts the velocity head behind the shock wave passing from the cylinder into power. The design approach in concept is to create the lowest back pressure on the engine while producing sufficient power for the turbo compressor. The design is complicated because most engines must operate over a wide load range and RPM range. At heavy load, there is more energy in the exhaust stream than is required, so a waste gate diverts the exhaust around the turbine. Attempts to use this flow to power an additional turbine are seldom practical because much of the pressure is lost in the exhaust shock wave and the full load condition is intermittent. The cylinder outlet pressure being kept at the fully expanded pressure in the cylinder but when the exhaust valve is closed would leave hot gases in the cylinder at higher pressure than the inlet condition for the cylinder. That condition is not practical for the engine.
The positive displacement device can be employed as the turbocharger in essentially the same manner. The first obstacle is the device has in the past been much larger than the turbine it would replace. A relatively large device will absorb and need to dissipate a great deal of heat. Expansion of larger parts at increased temperature and survivability of the mechanism then also becoming a major factor. Finally, carbon deposits and oxidation within the chamber over the life of the engine are detrimental. The creation of a much smaller positive displacement rotary machine for this application with thermal protection of lubricated surfaces overcomes much of this. The next to overcome is similar in nature to the turbocharger in that consideration of the back pressure on the cylinder must be balanced with efficient expansion of gases and the exhaust shock wave conversion to power. The shaft of the positive displacement device can be coupled to the engine and stepped up and down in speed, a waste gate can be employed, a generator can be coupled, and the device can have a number of schemes to control the flow. The shock wave for example will carry a charge of gases into the chamber if the rotor is somewhat beyond top dead center then expand these before being able to flow back towards the cylinder. The range of exhaust pressure for different engine loads and wide RPM range is difficult to control.
The cyclic nature of the rotary machine in contrast allows for a different approach than the exhaust turbine. For example, the exhaust charge from the piston engine can be fully expanded if the expansion is timed with the movement of the piston of the engine, which allows one expandor to handle four pistons if rotating at twice the RPM of the engine. Additionally, the inlet to the expandor is very close or attached directly to the exhaust manifold. Assuming in theory that a compact device with the ability to survive the life of the engine existed, then one could speculate as to the behavior for the gasoline, diesel or hydrogen powered engine. The device for a naturally aspirated engine could for example have two or three times the volume of one of the four cylinders, and an engine running with an inlet compressor would have a commensurately larger volume expandor. The condition then exists that under low load for the engine, there is too much of a vacuum pulled on the cylinder. Control for this scheme however is in concept simpler than the first approach discussed, and some of these approaches are as follows.
An expandor having a variable expansive volume is one method of accommodating the range of cylinder outlet pressures resulting from different engine loads. This can be achieved in a number of fashions, but the most straightforward is to open a flow channel in the rotary expandor outlet that is at midway through the expansion cycle. This will route flow from down stream of the outlet back into the expandor. Another method is to use a check valve on the expandor outlet. This allows the gases in the expandor chamber to expand to lower pressure than the ambient pressure then repressurize. That is to say the gases are further expanded to negative pressure in the device then repressurize to exit via a reed valve. The gases can even dissipate heat at negative pressure.
The device would not create the constant back pressure of turbo machinery and in theory the overall cycle is more efficient. This is closer to an ideal internal combustion engine cycle that calls for expansion to inlet temperature, which is at far lower pressure than inlet pressure, then recompressing to ambient pressure at the inlet temperature. Ideally, the overall cycle would resemble an open Stirling cycle.
Another method, which has received very little attention, is to actually pull a vacuum on the outlet of the expandor with a turbo compressor for example. The gases from the outlet of the expandor are cooled as much as is practical before recompression. A slight variation of this theme is to put the exhaust turbine on the expandor outlet and use this before the heat exchanger to power the exhaust turbo compressor. What has described from the standpoint of the hardware is very similar to an advanced exhaust gas recirculation pumping system for a diesel engine.
The applications are too numerous to list, but sufficiently small volumetric displacement rate machines for low-pressure applications are useful for quiet operation or where nearly constant pressure over a wide RPM range is desirable.